Communication satellites and other spacecraft typically employ a protective blanket to shield electrical and other componentry from an environment through which the spacecraft travels. This environment includes high and low temperature extremes, solar radiation, electron bombardment, and radio frequency interference from a variety of sources.
A conventional protective blanket, such as one described in U.S. Pat. No. 4,489,906 to Fellas, is constructed from a polyimide sheet upon which a coating of aluminum is deposited. The blanket is fastened to the spacecraft using rivets and/or adhesives. Although the blanket acts to reflect radiation, the blanket also acts to generate passive intermodulation (PIM) distortion.
Passive intermodulation distortion can be generated due to microdischarges by small cracks or voids in the aluminum coating. Also, the existence of metal-insulator-metal junctions that are exposed to carrier power can result in a non-linear behavior that can, in turn, result in passive intermodulation. Further, short circuits through the aluminum coating, and contact of the aluminum coating with connecting rivets and other parts of the spacecraft can act to generate passive intermodulation.
Currently-available flight-approved materials for protective blankets and shields fall into two categories. A first category includes conductive foils and thin film materials with a surface resistance less than 1.5 ohms per square. The first category of materials disadvantageously acts as a potential source of PIM. A second category of materials includes carbon-filled and thin film materials with surface resistance greater than 200 ohms per square. The second category of materials disadvantageously fails to adequately shield many sources of PIM.
These materials have been combined to achieve low PIM in thermal blankets and PIM shields. However, the combination of the materials has been disadvantageous in at least one of: (i) adding weight to the blanket or shield; (ii) increasing the material cost and/or labor cost to produce the blanket or shield; (iii) adding bulkiness and stiffness to the blanket or shield; (iv) resulting in a blanket or shield with poor thermal properties; and (v) providing only a marginal reduction in PIM.